Normally, fuel injection systems comprise at least one fuel injector controlled by a metering servo valve, which comprises a control chamber supplied with pressurized fuel. An outlet passage of the control chamber is normally kept closed by an open/close element via elastic means. The open/close element is actuated for opening the servo valve, by an armature of an electric actuator acting in opposition to the elastic means, for controlling an injection of fuel. The fuel injection system also comprises a unit for controlling the electric actuator, which is designed to issue for each fuel injection a corresponding electrical command.
In order to improve the performance of the engine, from EP1795738, a fuel injection system is known in which, for each fuel injection in a cylinder of the engine, the control unit issues at least one first electrical command of a pre-set duration for generating a pilot fuel injection, and a subsequent electrical command of duration corresponding to the operating conditions of the engine for controlling a main fuel injection. In some examples, the two commands are separated by a time interval such that the main fuel injection starts without a solution of continuity with the pilot fuel injection, i.e., such that the diagram of the supply of fuel during the fuel injection phase or event will assume a humped profile. In some examples, the two commands are separated by a time interval such that the main fuel injection starts without any solution of continuity with the pilot fuel injection, i.e., such that the diagram of the supply of fuel during the fuel injection phase or event will assume a humped profile.
Given the same duration of the electrical commands for the actuation of the pilot fuel injection and of the main fuel injection, the total amount of fuel introduced into the combustion chamber via the pilot fuel injection and the main fuel injection varies as a function of the time interval between the two aforesaid commands issued by the control unit. In particular, it is possible to identify two different modes of behaviour of the injector as a function of the time interval that elapses between the command for the pilot fuel injection and the command for the main fuel injection. In fact, it is possible to identify a limit value for said interval, above which the amount of fuel injected during the main fuel injection depends, not only upon the duration of the electrical command, but also upon the oscillations of pressure that are set up in the intake duct from the rail to the injector, on account of the pilot fuel injection.
For durations of the interval between the two fuel injections shorter than this limit value, instead, the amount of fuel introduced during the main fuel injection is affected by numerous factors, among which the duration itself of said interval, the train of rebounds of the open/close element, the evolution of the fuel pressure in the control chamber, the position of the needle of the nebulizer at the instant of start of the command for the main fuel injection and again the fluid-dynamic conditions that are set up in the proximity of the sealing area. In addition, the state of ageing of the injector, insofar as the wear of the parts in fluid-tight contact or in mutual motion, with extremely small coupling play, significantly affects the mode of rebound of the open/close element.
This phenomenon is substantially due to the presence of the pilot fuel injection, which in effect alters the fluid-dynamic conditions of the injector at the moment of the command for the main fuel injection. In particular, the limit value of the duration of the interval that separates these two modes of behaviour is approximately 300 μs.
In addition, the robustness of operation of the injector is markedly jeopardized when the time interval between the commands of the two fuel injections occurs below the limit value defined previously, and in particular when said interval becomes very small so that the pilot fuel injection interferes to a greater extent with the subsequent main fuel injection.
Notwithstanding the fact that it is possible to program the control unit so as to vary this interval between the pilot fuel injection and the main fuel injection during the service life of the injector, it remains in any case impossible to predetermine the degree of the correction to be introduced to cause the profile of the two fuel injections to continue to be humped.
The drawback encountered in the known fuel injection systems of the type described is due to the fact that, in order to obtain an injection profile of the humped type, it is necessary to set a value of the interval between the pilot fuel injection and the main fuel injection that is very small. Consequently, the start of re-opening of the servo valve for the main fuel injection occurs when the injection dynamics of the injected fuel is markedly variable and dependent upon the parameters set forth previously, with deleterious effects on the efficiency of the engine and on the pollutant emissions of the exhaust gases. These drawbacks increase rapidly following upon wear of the parts of the servo valve.